Technical Field
The present invention relates to a method and an apparatus for detecting gas leakage from a radioactive material sealed container. More specifically, the present invention particularly relates to a method and an apparatus for detecting leakage of inactive gas such as helium filled in a metallic canister of a concrete cask.
Related Art
A concrete cask has been a focus of constant attention as a storage means for high radioactive material represented by spent nuclear fuel in a nuclear reactor. The concrete cask is formed of: a cylindrical sealed container made of stainless steel and having a structure that stores spent fuel and seals the same by welding (hereinafter referred to as canister); and a non-sealed concrete-made storage container that has a shielding function and houses the canister (hereinafter referred to as concrete container). The concrete cask is a dry storage facility adapted to remove decay heat of spent fuel contained inside the canister by naturally convecting external air through an air ventilation port provided at upper and lower portions of the concrete container.
The canister has a sealed structure obtained by welding so as not to leak sealed radioactive material to the outside, and also adapted to transfer the decay heat of the spent fuel contained inside the canister via helium to the canister by sealing helium that is inactive gas having thermal conductivity higher than air. Therefore, in the event of helium leakage, there may be a concern that contamination caused by leakage of the radioactive material and insufficient heat removal of the decay heat occur.
In the case of assuming that the concrete cask is installed near coast, cooling air contains salt. Therefore, there may be a concern that a sealing function of the canister is lost by stress corrosion cracking. Additionally, in the case of storing the concrete cask in an inland area also, a deterioration/degradation problem cannot be entirely ignored in consideration of long-term storage, and there may be a concern that helium sealed inside the canister leaks due to a defect, corrosion, and the like at a welding portion of the canister.
A phenomenon of helium leakage is an event to be avoided because radioactive material may be emitted to the environment. Therefore, in the event of such a phenomenon, immediately detecting the event and taking countermeasures are needed. Accordingly, development of a technology to detect helium leakage at an early stage is demanded.
In response to this demand, there is a proposed method of detecting helium leakage, in which a temperature difference between a center temperature at a bottom portion and a center temperature at a top portion in a canister is monitored, and in the case where the temperature difference is increased and a feeding air temperature is decreased, occurrence of gas leakage is determined (JP 2005-265443 A).
However, according to the technology disclosed in JP 2005-265443 A, since it is necessary to measure the temperatures at two points of the top portion and the bottom portion of the canister housed inside the concrete container, construction work for installing thermocouples at the two points of the top portion and the bottom portion of the cask is required. However, depending on a structure of an air inlet port of the concrete cask, the construction work to directly install the thermocouple at the bottom portion of the canister may be difficult.